Technology update

Jun 12, 2014

Doped defects tune graphene for electronics

Despite the impressive headline characteristics predicted for graphene, it has proved tricky to harness its electronic properties for real-world device applications. Researchers in Brazil have now found a way to tune the material’s properties for electronic devices by combining nitrogen doping with certain defects generated during large-scale graphene fabrication.

"We have found a material based on graphene that may present a bandgap suitable for electronic applications," says Ricardo Kagimura of the Federal University of Uberlândia, one of the researchers leading the project. Defects are often thought to encroach on the wonder properties of pristine graphene but, as Kagimura points out, electronics applications need semiconducting materials with a suitable bandgap. "In our studies we observe that the incorporation of nitrogen atoms at the defective region in graphene opens a bandgap," he explains.

The investigations, which resulted from a collaboration between Kagimura’s group and the Federal University of Minas Gerais, reveal that nitrogen atoms readily substitute for carbon atoms at these defect sites. What is more, the nitrogen doping not only dictates whether the material is metallic or semiconducting, but even allows the width of the energy gap to be tuned.

Disoriented ways

So what led the researchers to stumble upon these tunable systems? Kagimura explains that large-scale production of graphene leads to grain-boundary defects, lines of imperfections that form between regions of crystalline graphene that have different orientations. Other research groups have also incorporated nitrogen into graphene structures, and Kagimura and his colleagues decided to put these two observations together.
The researchers modelled strips of defects in graphene where pentagons and heptagons replaced the hexagons in the honeycomb lattice at the boundaries between differently oriented regions of graphene. They then calculated energetically favourable structures and simulated scanning tunnelling microscope measurements of the structures for different concentrations of nitrogen dopants.

The calculations revealed some surprising properties. They showed that metallic or semiconducting stripes are produced within the graphene depending on the concentration of nitrogen atoms. Altering the distance between the defect lines also modifies the size of the energy gap in the material.

Real devices

An important question now is just how easily these structures can be produced. "Growth of graphene with these defects is possible and reported in the literature," Kagimura tells nanotechweb.org. "I believe that the main challenge is the incorporation of the nitrogen atom at the defective region, and the control of the distance between the defects, which is key for tuning the bandgap."

Further calculations

The calculations exploited the SIESTA code, which predicts the physical properties of a collection of atoms from first principles quantum mechanics. "Our research group has been investigating this kind of graphene system for the past few years," adds Kagimura.

For their future research Kagimura says there are many other two-dimensional materials to simulate and study. These include silicene, which is like graphene with the carbon atoms replaced by silicon. "Our studies can be used to explain experimental results, or to investigate unknown properties of known/new materials that can be confirmed by experiment," he adds. "It's a very interesting line of research."